The Celtics had good but unsustainable performances against the Kings. In the end, Boston built a 9-point lead in the third quarter and was outscored by 17 points in the fourth quarter before falling 114-97, which was its most lopsided loss of the season.

The Celtics are 8-7 in their last 15 games, a discouraging record for a team that remains the favorite to win another NBA title in June. It’s hard to see if there are worrisome deficiencies or if this team is just waiting for the games to become more meaningful. Regardless, it was gruesome at times.

“We have to continue to work on this as a group,” Jaylen Brown said. “It’s part of the journey. It may not be as beautiful as some would like. But I believe in this team. I believe in these guys and think we’ll be fine.”

The Celtics made just 11 of 41 3-point shots and said those offensive woes put tremendous pressure on their defense. However, they also showed an unusual lack of effort over several sections of the route.

With nine minutes left in the fourth and Boston trailing by just five, a defensive miscommunication between Brown and Jrue Holiday led to Malik Monk leaving a 3-pointer wide open from the top of the key while both were down court. Brown raised his palms to express confusion or frustration, and the Celtics were lucky the shot missed.

But the ball bounced to Sacramento’s Domantas Sabonis, who happily accepted the easiest of his 28 rebounds, a record for an opponent at TD Garden. Sabonis then shook off Holiday with a simple ball fake and had all the time he needed to line up his 3-pointer. This one went in.

Mazzulla took his first timeout of the quarter, and there were boos from fans who have had little reason to feel bad in recent years.

“I think if we don’t give it our all, I think that’s the most deserved boo,” Kristaps Porzingis said. “So it’s normal, and this is just a sign that they want us to raise our level and our energy, and we have to respond to that.”

Mazzulla acknowledged that there were individual areas of insufficient effort, but stressed that he did not believe these were related to larger problems.

“I would say when you have an inefficient offense, it puts pressure on the defense,” he said. “So I don’t think it’s a long-term effort. I think you hold a team like this to a 19-point quarter and a 23-point quarter, and you have two 21-point quarters, I think it just takes a toll on you.”

Brown had 28 points to lead the Celtics and Porzingis added 22. Jayson Tatum had 15 points, 12 rebounds and 8 turnovers, including five in the third quarter.

Sabonis blasted the Celtics, using his massive rebounding total to secure 23 points to go. The eight that he grabbed at the end of the offense gave the Celtics the most trouble.

“They just killed us on the glass,” Mazzulla said. “They wanted more possessions on that end of the floor. They got more offensive rebounds than we did, and from that perspective they wanted it more, so they were able to gain the shooting advantage there.”

DeMar DeRozan added 24 points for Sacramento, which was missing leading scorer De’Aaron Fox and has won six straight games under interim coach Doug Christie.

With the score tied at 76 early in the fourth quarter and Tatum on the bench, Kings rookie Devin Carter, who had appeared in just three games this year, scored eight straight Sacramento points to help his team to an 84-point lead -78.

Sabonis’ wide-open second-chance 3-pointer further discouraged the Celtics, and things got worse before they got better for Boston. Two 3-pointers from Keegan Murray continued the momentum, the second putting the Kings ahead 97-83 with 6:30 left. The Celtics never threatened again.

As the season nears its halfway point, Boston sits 6½ games behind the Cavaliers for first place in the Eastern Conference.

“We’ll be fine,” Porzingis said. “We’ll find out. I’m telling you, I’m confident we’ll get back on our feet. We’re not having the best moment as a team right now, and that’s completely normal… Believe me, we’ll get through it.”